The present invention relates to the field of catalyst supports, and specifically to those supports which can be used to make catalytic converters for automobiles. The invention can also be used to make catalyst supports for catalytic incinerators, which burn out pollutants in a stream of air, or for other applications of catalysis.
In a catalytic converter for an automobile, it is necessary to display the catalyst on a supporting structure, so as to maximize the contact between the catalyst and the exhaust gas. Catalyst supports made of ceramic materials have been known for a long time, and were known before metal supports were developed. But supports made of thin metals have the advantage that they can be made in large cross-sections, as would be needed for treating large gas flows. Another advantage of metal is that the walls of the support can be made thinner, thereby giving a higher open frontal area, and a lower pressure drop through the support. In addition, for metals which can be rolled down to a foil, it is comparatively easy to fabricate such metals into any shape desired.
Only a few metals have been usable as catalyst supports, in the prior art. A constant problem has been that many metals react chemically with the catalyst, causing the catalyst to lose its activity. Such reaction is especially likely at the high temperatures present in automobile exhaust gas.
Metal honeycomb catalyst supports are described in U.S. Pat. Nos. 4,162,993, 4,190,559, 4,247,422, 4,301,039, 4,350,617, 4,402,871, and 4,425,305. All of the above patents are incorporated herein by reference.
Metal-supported catalysts for use in automobile converters have been under development for about ten years. A metal catalyst support for an automobile converter must not only withstand high temperatures, which can approach 2000.degree. F., but must also withstand frequent heating and cooling, as well as vibrations from the automobile, and should, ideally, last for the life of the vehicle. Society of Automotive Engineers Paper 770299 (1977) describes the development of an automobile catalyst. In the cited paper, four metals were tested as catalyst supports. The criterion considered was resistance to oxidation at a temperature of 1085.degree. C., when the metal is rolled into a foil only 0.002 inches thick. The only metal to pass this test is known by the trade name Fecralloy, and has 4-5% aluminum, 15-20% chromium, 0.1-0.3% yttrium, the remainder being iron. Fecralloy is a product of the UK Atomic Energy Authority. Similar alloys are made in the United States, for example the alloy known as Kanthal A-1, made by Kanthal Corporation. Kanthal A-1 contains 5.5% aluminum, 22 % chromium, 0.5% cobalt, the remainder being iron.
When the above-described alloys are heated, the aluminum diffuses to the surface, where it forms an adherent self-healing aluminum oxide diffusion barrier. This barrier prevents further oxidation, so that it protects the metal core. The barrier also prevents, at least partly, base metal in the core from diffusing into the catalyst coating. The catalyst coating would be better protected if the barrier could be made thicker. It would seem natural to accomplish this by adding more aluminum to the alloy. However, at higher levels of aluminum content, the alloy splits or cracks when it is rolled into the thin foil that is needed for making honeycomb catalyst supports.
Another seemingly simple solution would be to make the catalyst support from aluminized steel sheet, which is unalloyed iron that is coated with aluminum. Upon heating, the aluminum is oxidized to aluminum oxide. Apparently, a barrier of any thickness could be formed by starting with a sufficiently thick coating of aluminum. In my experiments, this method failed because at high temperature, the iron core oxidized and caused the catalyst coating to spall off.
U.S. Pat. No. 4,247,422 describes a method of making a honeycomb from thin metal foil, rich in aluminum, without having to roll down an alloy that is rich in aluminum. The honeycomb is formed from a foil of substantially pure iron. Then aluminum and chromium are diffused into the iron by soaking the honeycomb in a lead bath that contains the aluminum and chromium in solution. The aluminum and chromium are not uniformly distributed throughout the base metal. Instead, there is a gradient in its chromium and aluminum content, with most of the chromium and aluminum being located in an outer layer.
To make a metal-supported catalyst, a base metal is coated with activated alumina (Al.sub.2 O.sub.3) having a surface area of about 50-200 m.sup.2 /g. The alumina is applied as a slip or slurry, which is dried and calcined to form the alumina coating. Then the alumina coating is impregnated with the catalyst metal, usually a platinum group metal.
The procedure for applying the alumina and the catalyst to the metal support is well known in the prior art, and is used regardless of the composition of the metal support. The difficulty lies in selecting a composition for the base metal, such that the base metal does not interact with the impregnated catalyst and destroy its activity. At temperatures above about 900.degree. C., the base metal of the support can diffuse into the alumina coating and react with the coating, and also with the platinum metal supported thereon. The base metal accelerates sintering of the alumina coating, whereby the surface area of the alumina coating is reduced. Also, the base metal forms alloys with the platinum metal, which alloys have less catalytic activity than the unalloyed platinum metal. The present invention therefore is concerned with providing a composition which can be used as a metal support.
An example of a metal catalyst support is described in Japanese Patent Application No. 49-99982. The latter reference discloses a support consisting of an aluminum-clad iron sheet, and another support made with 18-8 stainless steel, also coated with aluminum. Both supports are then coated with a combustion catalyst. The reference contains no hint of the temperatures at which the support was tested. In addition, we have found that aluminum-coated 18-8 stainless steel cannot be rolled down to foil thickness.
Another example of a metal catalyst support is given in U.S. Pat. No. 3,873,472. This patent, like the Japanese application discussed above, discloses supports having a base metal of substantially pure iron, or a base metal made of 18-8 or 18-12 stainless steels.
The present invention provides a metal catalyst support wherein the base metal does not react with the catalyst, and wherein the catalyst retains its activity over a long period of time, and at very high temperatures. The metal catalyst support of the invention can also be formed into the thin foil which is needed to make catalytic converters for automobiles.